Leadbelcher vs Speed Metal

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Speed Metal is noticeably lighter, more saturated than Leadbelcher.

Can I use Speed Metal instead of Leadbelcher?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Speed Metal is noticeably lighter, more saturated than Leadbelcher.

Can I use Leadbelcher instead of Speed Metal?

The same difference applies the other way round: Leadbelcher is noticeably darker, more muted.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.